1. Field of the Invention
This invention resides in the field of diagnostic methods and pharmaceutical agents associated with Alzheimer's disease.
2. Description of the Prior Art
Alzheimer's disease (AD) is the most common form of dementia, afflicting tens of millions of people worldwide, and with the high vulnerability of the elderly to this disease, the number of cases is increasing each year as life expectancy increases throughout the world. The debilitating characteristics of AD include irreversible memory impairment, continuous cognitive decline, and behavioral disturbances, and the disease presents a continuing challenge for society and for health care systems. Advances have been made in understanding the etiology, genetics, and pathophysiological mechanism for AD, but no cure has as yet been found.
It is known that AD is associated with the presence of senile plaques of amyloid β (Aβ) peptide and neurofibrillary tangles composed of hyperphosphorylated protein tau. The plaques are formed by the aggregation of the Aβ peptide, and the peptide itself is produced by the sequential proteolytic processing of amyloid precursor protein (APP) by β and γ secretases. Plaque formation is preceded by the accumulation of Aβ due to genetic mutations. A high level of Aβ is thus regarded as an initial pathogenetic factor for the development of neurodegeneration in AD.
Current efforts to control or prevent the development or progression of AD are directed toward prevention of Aβ plaque formation, primarily through the use of drugs that inhibit the aggregation of Aβ. Recent reports of such drugs appear in Bose, M., et al., “‘Nature-inspired’ drug-protein complexes as inhibitors of Aβ aggregation,” Biochem. Soc. Trans. 2005, 33 (pt. 4), 543-547; Gestwicki, J. E., et al. “Harnessing Chaperones to Generate Small-Molecule Inhibitors of Amyloid β Aggregation,” Science 306, 865-869 (29 Oct. 2004); Braun, P. D., et al., “A Bifunctional Molecule That Displays Context-Dependent Cellular Activity,” J. Am. Chem. Soc. 2003, 125, 7575-7580; Briesewitz, R., et al., “Synthetic Bifunctional Molecules Containing a Drug Moiety and Presenter Protein Ligand,” U.S. Pat. No. 6,372,712 B1, issued Apr. 16, 2002; Briesewitz, R., et al., “Bifunctional Molecules Having Modulated Pharmacokinetic Properties and Therapies Based Thereon,” International Patent Application Publication No. WO 01/35748 A1, publication date May 25, 2001; and Briesewitz, R., et al., “Targeted Bifunctional Molecules and Therapies Based Thereon,” International Patent Application Publication No. WO 01/35978 A1, publication date May 25, 2001.
The use of these drugs is most effective when combined with the detection of amyloid deposits prior to plaque formation, together with detection of the tissues in which the deposits reside. The relation of the tissues in which the amyloid deposits reside to the stage of the disease is described by Braak, H., et al., “Neuropathological staging of A beta deposition in the human brain and its relevance for the development of AD,” J. Neural Trans. Suppl. 1991, 82, 239-259, who report that amyloid deposits first appear in the basal neocortex, then spread to all areas of the cortex, and subsequently to subcortical brain regions including the cerebellum. If the deposits are detected in the basal cortex, therefore, the proper administration of drugs can prevent progression to the other areas of the cortex and of the brain in general.
The existence and location of amyloid deposits has been detected by conventional imaging techniques, notably positron emission tomography (PET) and single photon emission computerized tomography (SPECT), enhanced by the administration of imaging agents that demonstrate binding specificity for Aβ. Agents that have been developed for this purpose are radiolabeled antibodies to Aβ and radiolabeled peptide fragments, as well as small molecules such as derivatives of Congo red (sodium salt of benzidinediazo-bis-1-naphthylamine-4-sulfonic acid), thioflavin, stilbene, and acridine. The use of these imaging agents is reported by Nordberg, A., “PET imaging of amyloid in Alzheimer's disease,” The Lancet, Neurology 3, 519-527 (September 2004). Prominent among these agents are 2-(1-{6-[(2-[18F]fluoroethyl)(methyl)amino]-2-naphthyl}ethylidene)malononitrile (18F-FDDNP) and 2-(4′-[11C]methylaminophenyl)-6-hydroxy-benzothiazole (11C-PIB).
The effectiveness of small molecule imaging agents is limited, since their small size allows them to easily penetrate the blood-brain barrier, causing these agents to have a wide biodistribution. As a result, these agents remain in the tissue of interest for only a short period of time, too short in many cases for effective detection of Aβ. They are therefore useful primarily for the detection of plaques, rather than pre-plaque Aβ proteins.
By binding to Aβ, these imaging agents act in a manner similar to the drugs referenced above. A notable distinction that is evident from the publications set forth above is that the inhibitory effect of the drugs has been enhanced by combining the drugs with chaperone proteins. Chaperone proteins, which are endogenous to many tissues, are widely known for providing beneficial cellular effects due to their ability to bind to various types of normal and abnormal proteins, including those associated with neurodegenerative diseases. Indeed, one natural function that certain endogenous chaperone proteins serve by their binding ability is to prevent protein aggregation. In the context of drug administration as described in the above-referenced publications, the chaperone protein serves a dual purpose—it increases the affinity of the drug for Aβ (in this capacity the chaperone protein is referred to as a “presenter protein”) and it blocks a large portion of the contact surface of the Aβ, thereby adding to the steric inhibition that the drug itself creates. The drug and chaperone are joined by a bifunctional molecule of which one functionality binds to the drug and a second functionality binds to the chaperone. To allow the chaperone to complement the drug effectively, the bifunctional molecule must have a high binding affinity for both the drug and the chaperone.
One might therefore apply similar reasoning to the imaging problem and combine the imaging agent with a chaperone protein. The added bulk achieved by the binding of the chaperone might theoretically reduce the undesirable rapid transit of the imaging agent through the tissue of interest, and the added affinity to the target due to the attachment of the chaperone might increase the binding affinity of the agent and chaperone together to pre-plaque Aβ, thereby increasing the detectability of the Aβ proteins. Accordingly, one might contemplate taking bifunctional molecules proposed for the therapeutic inhibitory effect, derivatizing the molecules by adding a detectable, e.g., radioactive, label, and using them for imaging. Effective imaging is not well served, however, by a bifunctional molecule that is effective for inhibition of plaque formation. Since the bifunctional molecule for effective inhibition binds strongly to both the target (Aβ) and the chaperone, the label will appear on three different complexes—the complex formed by the target and the bifunctional molecule alone, the complex formed by the bifunctional molecule and the chaperone alone, and the complex formed by all three, i.e., the target, the bifunctional molecule, and the chaperone. One will then be unable to distinguish between these three complexes and thereby detect the presence of Aβ by simply detecting the label.